1. Field of the Invention
This invention relates to supervisory and control systems which employ electrical power distribution lines as a communication link between a transmitter and one or more receivers, and more particularly, to a supervisory and control system including a multichannel transmitter programmable to transmit control pulses in different data channels of a data frame and receivers which are programmable to respond to control pulses in preselected data channels.
2. Description of the Prior Art
Various supervisory and control systems which employ existing electrical power distribution networks for the transmission of information have been proposed for applications including data acquisition, condition monitoring and the control of functional devices, the latter being the most predominant use for such systems. In systems for controlling functional devices, one or more transmitters are operable to provide control signals which are impressed on the power lines and transmitted over the power lines to receivers. The receivers respond to the control signals to effect energization of an associated device, such as a light, a motor or an alarm indicator. The control signals are generally coded to permit selection of a given receiver for enabling operation of a selected device.
Frequency coding techniques are the most common method for providing selective addressing of the receivers. The operating frequencies for known systems range from 255Hz to 400KHz with the majority of systems operating in the 200 KHz range. The systems which employ frequency coding are characterized by various shortcomings. For example, the use of high signal levels for the control signals which are coupled to the power lines to enhance reliable communication usually interferes with the operation of electrical appliances connected to the power lines, and also increases the cost of the system transmitters. The use of low signal levels increases the system susceptibility to electrical noise unless highly sensitive receiver circuits are used in the system. The use of such receivers result in a considerable increase in the cost of the receivers.
Some of the systems have proposed the use of AC power signals to synchronize the operation of the transmitters and receivers. However, the performance of most systems has not been entirely satisfactory primarily due to ever changing impedance characteristics of the power distribution network, phase shift, or signal attenuation for when long transmission distances are involved.
In such applications a further consideration is that most power distribution systems include a multiphase distribution network, and it is generally desirable to communicate from a transmitter connected to one phase of a power transformer to a receiver connected on a different phase of the same power transformer. Signal attenuation in these cases is extremely high, thereby making communication from one phase to the other phase highly unreliable and most difficult to accomplish. It may also be necessary to operate such systems in large buildings where several power transformers may be used to supply the electrical power. In such cases, it is imperative that a provision for transmission of signals from one circuit located on the secondary side of a power distribution transformer to another located on the secondary side of a second distribution transformer is available.